Method of producing stoichiometric mg-al spinel crystals for integrated circuits

ABSTRACT

A method of producing Mg-Al spinel crystals of a stoichiometric composition free from precipitation and tension. The invention is characterized in that the carrier crystal is an alumina-rich spinel crystal. The crystal growing thereon is produced according to its length by changing the provided mixing ratio of Mg0/Al203 to a stoichiometric composition of the desired spinel crystal. During the growth process, slight amounts of Ti02 are added to the spinel powder supplied.

O United States Patent 1111 3,619,131

[72] Inventor Josef Gra m i r 150] Field of Search 23/301, Unterhaching, Germany 304, 305, 52; 252/632, 63.5, 520, 52l l2l] Appl. No. 824,734 {22] Filed May 6, 1969 I56] References Cited Patented 9. 1971 UNITED STATES PATENTS [731 Assignee Siemens Aklienflmllscha" 3,226,l93 l2/l965 13011011 23/191 Berlin and Munich Germany 3,457,033 7/l969 Catti et al. 23/52 1 1 Priority May 6,1968 3.472,6l5 l0/l969 Wang 23/52 [33] Germany h [3| P l 17 67 394 Primary Lxanuner-John T. Goolkaslan Ass/11mm Erumim-r- Lorraine T. Kendell Allorneys-Curt M. Avery, Arthur E. Wilfond, Herbert L.

Lerner and Daniel J. Tick [54] METHOD OF PRODUCING STOICHIOMETRIC MG'AL SHNEL CRYSTALS FOR NTEGRATED ABSTRACT: A method of producing Mg-Al spinel crystals of CIRQUITS a stoichiometric composition free from precipitation and ten- 7 Clamms Drawmg Flgs' sion, The invention is characterized in that the carrier crystal [52] U.S. Cl 23/52, is an alumina-rich spinel crystal. The crystal growing thereon 23/30l, 23/304, 23/305, 252/632, 252/635, is produced according to its length by changing the provided 252/520, 252/52l mixing ratio of Mg0/Al 0; to a stoichiometric composition of [51] lnt.Cl B0lj l7/24, the desired spinel crystal. During the growth process, slight amounts of'l'iO are added to the spinel powder supplied.

PATENTEDHUV i n 3.619.131

SHEET 1 OF 2 I I I "H M im I PAItmEum 9 Ian 3,619.131

SHEET 2 BF 2 METHOD OF PRODUCING S'IOICIIIOMETRIC MG-AL SPINEL CRYSTALS FOR INTEGRATED CIRCUITS Considerable advantages have been seen for solving insulation problems in integrated circuits by epitactically precipitated semiconductor layers on highly insulated monocrystalline substrate layers. The use of sapphires or spinel as substrate layers is known. Several factors speak for the use of spinel. In contrast to sapphires they have the same cubic lattice symmetry as silicon. Furthermore, they are not very hard, so that they are easier to process. The disadvantages of spinel, which is produced in accordance with the Vemeuil method, is its dissociation during the cooling process in the Vemeuil furnace and subsequent temperature treatments. Up to now, monocrystalline spinel can be industrially produced at a mixing ratio only of MgO/Al of 1:2 to 1:4. These spinel which are rich in alumina are strongly tensioned. After cooling in the Verneuil furnace, they are in a state of preprecipation. During a subsequent heat processing which is necessary during annealing and epitactic coating, this preprecipation via a metastable intennediate structure leads to final precipitations of A1 0 These final precipitations occur mainly at the surface and may therefore cause, in the grown semiconductor layer, stability failures, twin formations or even a polycrystalline degenerated growth. These A1 0,, seeds prefer to form inside the crystal at the optically visible places and produce, thereby, an additional tension in the substrate discs.

The present invention relates to a method of producing spinel crystals of stoichiometric composition, free of precipitation and tension. more particularly of substrate wafers, to be used in the production of epitaxial layers comprised of semiconductor material, according to the so-called Verneuil method, whereby finely distributed Mg-Al spinel powder is contacted with the heated or molten dome of a carrier crystal caused to cool and crystallize upon the dome and removing the carrier crystal from the heated zone, according to the solidification speed.

We use an alumina-rich spinel crystal as a carrier crystal. The crystal grown upon the latter is produced along its length, by changing the mixing ratio of MgO/Al 0; to a stoichiometric combination of the desired spinel crystal, whereby slight additions of Ti0 are added during the growth process to the spinel powder provided.

The method of the invention affords the opportunity to produce by mechanical or thermal processes, stable Mg-Al spinel which, because of their highly insulating properties, can be used as substrate bases for the production of epitaxial semiconductor semiconductor materials.

Due to their high melting temperatures (approximately 2,100 C.) only spinel produced according to the Verneuil method can be used today for the above-indicated purpose. As it was not possible, heretofore, to produce in accordance with said method, the thermally stable alumina-poor deprived spinel at a size required with sufficient mechanical stability, only the aluminal-rich spinel could be used as substrate wafers because of their good mechanical stability. However, they are very unstable with respect to heat processing. According to previous experiences, the alumina-rich substrate wafers dissociate during heat treatments, and this disturbs the epitactic growth process of semiconductor materials and consequently considerably disturbs the electrical properties of the layers. This problem was solved by the method of the present invention, and aluminapoor Mg-Al spinels or spinel crystals with stoichiometric combination are now also available for epitactic growth processes.

A further development of the invention is to use a Mg-Al spinel crystal, composed approximately of Mg0/Al 0 at a ratio of l:3.l as a carrier crystal.

It is also possible according to the invention, to select a processing method whereby the A1 0 content of the provided spinel powder diminishes gradually up to a stoichiometric composition. According to a particularly preferred embodiment of the present invention, it is advantageous to adjust, at first, a mixing ratio of MgO/Ahfl of 1:3.1 then a mixing ratio of l:2.5 and 1:1.7 and, finally, a mixing ratio of H. The last stage of the growth process then corresponds to the stoichiometric composition of the desired spinel crystal.

It lies within the framework of the invention to add small amounts of TiO,, to the spinel powder. An addition of about 0.1 percent Ti0 was found to be particularly advantageous. This slight addition of Ti0 to the original material helps to diminish the tension in the grown Mg-Al spinel crystals still further.

If necessary, the section of the grown spinel crystal can be separated from the remaining spinel crystal, including the carrier crystal, following the growth process and can be used directly as a substrate material for epitactic growth processes. The original crystal seed can be used again as a carrier crystal seed for a new production process, after having been separated from the pulled crystal.

The method of the present invention is particularly well suited for the production of spinel crystals which are used as substrates for epitactic precipitation of semiconductor materials, particularly of silicon. The integrated circuits produced in these silicon layers are characterized by particularly stable and good electrical characteristics.

The invention will be described with respect to the drawing, in which:

FIG. 1 shows a crystal produced according to the invention; and

FIGS. 2 and 3 show apparatus for producing such crystal.

FIG. I shows an Mg-AI spinel crystal, produced according to the method of the present invention. Region 30 represents an alumina-rich MgAI crystal of composition I23. l used as both a carrier crystal and as the first layer, precipitated thereon which has the same composition. Region 31 is precipitated upon region 30 by using a spinel powder with a composition of l:2.5. Region 32 of the grown crystal corresponds to a powder composition of l:l.7 and region 33 represents the stoichiometric composition of Mg0/Al,0 at a ratio of 1:1.

FIG. 2 schematically illustrates a Vemeuil apparatus which is also used for other crystal growth processes, such as growing synthetic rubies. The funnel 1 holds the vibrating box 2 containing the original material 6, provided for the particular section to be produced and comprised, for example of a fine grained Mg-Al oxide powder of an initial composition ratio 1:3.l, followed by 112.5, then 121.7 and finally lzl. The particle sizes are smaller than 70 pm. The material is obtained by heating for approximately 2 hours in a quartz boat, ammoniaaluminum-alum and Mg-Al sulfate, at l,200 C. A metal sieve 3 (mesh size around pm.) is at the bottom of the vibrating box 2. The sieve is clamped on a ring (not shown), which can be screwed upon the vibrating box 2. A hammer 4 is mounted above the funnel l and is operated by a cam 5, driven by a motor. The hammer knocks against the vibrating box 2 filled with oxide powder 6, 60 to 120 times per minute. The filler neck for the respective powder mixture is seen at 28. A burner 7 is installed below the funnel 1. Oxygen, which flows in at arrow 8, is supplied to funnel I while hydrogen, flowing in at arrow 9, is delivered directly to the burner 7, through bores 20 which are also present in the burner pipe. The burner 7 protrudes somewhat with its mouth I7, into a cylindrical furnace 10, encased in an aluminum sheet 11. The furnace is approximately 250 mm. high, with an outer diameter of approximately 250 mm. and an inner diameter of 40 mm. and is sealed on the top with a cover plate 21 and on the bottom with plate 22 of aluminum.

The furnace 10 has an observation hole 13, through which the processes going on inside can be observed. To avoid impurities and to adjust the optimum thermal insulation, the furnace is lined with a casing 12 of fire-resistant ceramic adjusted to the diameter of the furnace. The space between the furnace jacket and the casing is filled in with loose sintered degussit granules or with a firebrick mass 23 with good heat insulation and a thickness of mm. The crystal holder 14 (a small rod or pipe of sintered alumina, degussit or spinel crystal) extends from below into the furnace and is mounted upon the spindle of a gear block which is movable in the perpendicular direction. The gear block serves for removing the grown crystal, and is either hand or motor operated.

At the onset of the process, a carrier seed crystal 30 comprised of an alumina-rich spinel (113.1) is mounted upon the crystal holder 14 and heated by the oxyhydrogen flame. Oxide powder 6, from the vibrating box 2, is delivered first at a composition of 1:13.! through the shacking funnel 18, which is additionally installed into funnel 1 to ensure control of a uniform powder supply to the oxyhydrogen flame and melted. Shacking funnel 18 is also sealed by a metal sieve l9 l60 um. mesh width). After a short time, the first section, also indicated as 30 in HO. 1, of the spinel rod 16, is obtained at approximately 1,950 C. The spinel is then permitted to continue its growth, if possible with a constant flame and a uniform powder supply, but with an alternating composition, until of desired length. In order to prevent the crystal from growing into the flame, the latter is lowered in accordance with its growth increase, and with the aid of a gear 15, at a pulling velocity of about 5 mm./hr. When the crystal reaches the desired composition or the stoichiometric composition, the powder supply is interrupted and the crystal holder 14, together with the grown crystal 16, comprising the four zones 30, 31, 32, 33, is removed from the heating zone of the Verneuil furnace, with the aid of the drive, either after the oxyhydrogen flame has been turned off or while the flame is burning. The transport and the composition of the reaction gas was effected according to known methods and based on the growth of synthetic rubies.

The refilling of the powder with small alumina contents, into the vibrating box 2 is effected by means of a device which is shown in the upper portion of H6. 3. By opening the upper clamp 24, the powder falls from the storage filling funnel into the filler tube 26. When clamp 24 is closed and clamp 27 opened, the powder to be refilled arrives, via the tiller neck 28 into the vibrating box 2. It is preferred not to add more powder of a specific composition, than is required for the buildup of a layer. The individual stages can be grown, with the aid of the aforedescribed refilling device, without any difficulty and without having to extinguish the flame. While the powder is changed, only the melting temperature which changes slightly according to the composition of the powder, has to be considered. The melting temperature of the spinel powder rises somewhat with a stoichiometric composition. The same reference numerals as in FIG. 2 apply to the remaining portions of FIG. 3.

I claim:

1. A method of producing Mg-Al crystals, of a stoichiometric composition, and free from precipitation and tension, more particularly spinel crystals, which comprises bringing divided Mg-Al spinel powder into contact with a heated or molten dome of a carrier crystal, and permitting the powder to cool and crystallize on said dome, moving the carrier crystal out of the heating zone in accordance with the melting speed of said powder, using as the carrier crystal an alumina-rich spinel crystal, and changing the compositions of the crystal growing in accordance with its length by changing provided mixing ratio of MgO/Al 0 to a stoichiometric composition of the desired spinel crystal and adding slight amounts of Ti0 spinel powder supplied during the growth process.

2. The method of claim 1, wherein an Mg-Al spinel crystal, composed of Mg0/Al 0 at a ratio of l:3.l is used as a carrier crystal.

3. The method of claim 1, wherein the A1 0 content of the spinel powder supplied is continuously reduced during the growth process, up to a stoichiometric composition.

4. The method of claim I, wherein first a mixing ratio of Mg0/Al,0 is adjusted to l:3.l then a mixing ratio of 122.5 and 1:1.7 and, finally, a mixing ratio of lzl.

5. The method of claim I, wherein the amount ofTit) is approximately 0.l percent.

6. The method of claim 5, wherein the diameter of the grown spinel crystal et uals the diameter of the carrier crystal.

7. The method of c arm 6, wherein the section of the spinel 

2. The method of claim 1, wherein an Mg-A1 spinel crystal, composed of Mg0/A1203 at a ratio of 1:3.1, is used as a carrier crystal.
 3. The method of claim 1, wherein the A1203 content of the spinel powder supplied is continuously reduced during the growth process, up to a stoichiometric composition.
 4. The method of claim 1, wherein first a mixing ratio of Mg0/A1203 is adjusted to 1:3.1 then a mixing ratio of 1:2.5 and 1:1.7 and, finally, a mixing ratio of 1:1.
 5. The method of claim 1, wherein the amount of Ti02 is approximately 0.1 percent.
 6. The method of claim 5, wherein the diameter of the grown spinel crystal equals the diameter of the carrier crystal.
 7. The method of claim 6, wherein the section of the spinel crystal grown, which has the stoichiometric composition, is separated from the remaining spinel crystal. 